Passing method to a typescript decorator

Question

I am trying to pass a method to a typescript decorator. I am able to do so when I do this as an inline arrow function. But I am not able to do this in the following way:

function preProcessor(preFn:any) {
    
    return function (
        target: any,
        propertyKey: string,
        descriptor: PropertyDescriptor
    ) {        
        descriptor.value = function (param:any) {
            return preFn(param);
        }
    };
}

class C {
    @preProcessor(this.capitalize)
    methodA(str: any) {
        console.log(str)    }

  capitalize = (x:any)=>{return x.toUpperCase()}

}

var c = new C()
console.log(c.methodA("samurai..."))

This works:

@preProcessor((x:any)=>{return x.toUpperCase()})

This is not possible. When the decorator runs, there is no `this` yet. Your best bet would be to pass the *name* of the method, i.e. `@preProcessor('capitalize')` — Bergi, Sep 22 '20 at 22:17
Then you can call `this[methodName]()` inside the decorated method — Bergi, Sep 22 '20 at 22:24
That seems to be working. One noob question though- I believe it's possible because `this` would refer to the context of the caller. Which is `class C` right? Therefore it will work because capitalize is inside class C. Is that correct? — SamuraiJack, Sep 22 '20 at 22:28
No, [it refers to `c`](https://stackoverflow.com/q/3127429/1048572) or whatever you are calling `.methodA()` on. — Bergi, Sep 22 '20 at 23:24
@Bergi isn't that same as what I said? O.o? I mean `this` will hold scope of class c instance on which method was called, right? — SamuraiJack, Sep 22 '20 at 23:28
I was just taking you literally - the instance `c` is not the same as the class object `C`. And there is no "scope" for object properties. The object `c` inherits `methodA` from `C.prototype`, and has an own property `capitalize`. Notice you can also do `c.methodA.call({capitalize() { return x.toLowerCase(); }}, "Ooops")` where `this` does not refer to `c` and it will call a different capitalise function. — Bergi, Sep 22 '20 at 23:32

Passing method to a typescript decorator

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